Ferromagnetic state in ultrathin orthorhombic CrAs films: Thickness, lattice distortion, and half-metallic contributions Alexandre A. Araújo and Bernardo Laks Instituto de Física Gleb Wataghin, Universidade Estadual de Campinas, Caixa Postal 6165, 13083-970 Campinas, São Paulo, Brazil P. C. de Camargo Departamento de Física, Universidade Federal do Paraná, Caixa Postal 19091, 81531-990 Curitiba, PR, Brazil Received 5 October 2006; published 20 November 2006 Orthorhombic CrAs thin films were investigated using first-principles spin-polarized calculations in the full-potential linearized augmented plane-wave method. Our results consider two different geometry optimi- zation processes and show that the ferromagnetic state is favored by b-axis expansion, being more stable than the antiferromagnetic state for film thickness below approximately 24 Å. The calculated maximum magnetic moment per formula unit is near 3 B and decreases with increasing film thickness, in good agreement with the observed saturation magnetization. The electronic structure of very thin films with expanded b axis suggests a half-metallic behavior. DOI: 10.1103/PhysRevB.74.172411 PACS numbers: 75.70.-i, 71.15.Ap, 71.15.Mb, 71.20.-b The search for zinc-blende ZB half-metallic HM com- pounds among 3d transition metal chalcogenides is justified because of the high technological interest in the so-called spintronics, which requires polarized spin-injection de- vices. 1–3 A strong effort from the theoretical point of view, coupled to experimental evidence, has supported the view that ZB compounds of transition elements with atoms of groups V and VI may achieve epitaxial growth on semicon- ductors, preserving the HM characteristics for a few mono- layers thickness. 4–6 In spite of the theoretical prediction that ZB CrAs on GaAs is ferromagnetic with half-metallic char- acter, Etgens et al. found that CrAs grown on GaAs is ferromagnetic, 7 however, being orthorhombic near the inter- face. This orthorhombic ferromagnetic structure is mainly due to a b-axis deformation from 3.445 to 3.63 Å. Therefore, the main question to be answered concerns the origin of the magnetic behavior of orthorhombic CrAs films. This paper calculates the effects of lattice distortion and film thickness on the existence of ferromagnetism and half-metallic fea- tures. Ferromagnetic phase stabilization depends on the charac- teristics of film. thickness and constraints due to the substrate lattice, in addition to interfacial features. In fact, ferromag- netism occurs for Cr, CrAs, or even arsenized Cr films pre- pared on GaAs substrates. 8 It is worth noticing that the mag- netic moment per formula unit decreases as film thickness increases; it is quite generally accepted that the magnetic signal originates in the layer near the interface with GaAs. 7,9 The orthorhombic ferromagnetic phase stability of a MnP-type Pnma symmetry group CrAs thin film is inves- tigated, together with the magnetic moment and half- metallicity evolution as a function of film thickness and lat- tice parameters. We carried out energy minimization, within the scope of the Powell method, 10 as a function of all seven parameters lattice and internal, searching for a new ortho- rhombic CrAs phase, based on the known bulk orthorhombic parameters. 11 This was performed from first-principles spin- polarized electronic band-structure calculations, based on the density functional theory, 12 using the general gradient ap- proximation according to the Perdew-Burke-Ernzerhof parametrization. 13 For the self-consistent solution of the Kohn-Sham equations we have applied a full-potential lin- earized augmented plane-wave LAPW formalism, based on the WIEN code. 14,15 Relativistic effects are taken into account in the scalar approximation, but we have neglected spin-orbit coupling effects. The muffin-tin radii were chosen to be 2.3 Å for both Cr and As atoms. Inside the atomic spheres the charge density and the potential are expanded in spheri- cal harmonics up to l =6. The maximum l value for partial waves used inside atomic spheres is l max = 10. The nonspheri- cal potential contribution to the Hamiltonian matrix has an upper limit of l = 4. The plane-wave cutoff is RK max =7.0 and we have used around 100 k points in the first irreducible Brillouin zone. Self-consistency was considered attained when the difference in total energy per cell was smaller than 1  10 -7 Ry. Thin films of different thicknesses were considered in the geometry optimization processes adopting the orthorhombic structure, where the a and b axes of the CrAs are in the plane parallel to the GaAs001 surface and the c axes is perpen- dicular to the growth direction. This crystal orientation was chosen based on the experimental results found by Etgens et al. 7 Geometric and magnetic structures of orthorhombic CrAs, are shown in Fig. 1. In the Pnma symmetry group there are four Cr and four As inequivalent atoms. Thus, the ferromagnetic FM, Fig. 1a, and the three most likely magnetic orders involving Cr atoms, Figs. 1b–1d, were considered in the total energy minimization procedures. Among the antiferromagnetic AFM configurations, the lowest total energy corresponds to the configuration shown in Fig. 1c, and we called it the AFM state. We should mention that the paramagnetic phase corresponds to the highest-energy state. The surface effects on the CrAs thin film were taken into account by introducing a film-vacuum interface in a super- cell scheme, with a vacuum region corresponding to about six atomic layers of CrAs, considered large enough to pre- vent interaction between films. Substrate effects were as- sumed to constrain the a and b axes of CrAs to fixed values. Based on experimental evidence the possible interfacial re- action is considered to be negligible. 8 Bulk and ultrathin films of 8, 12, 16, and 32 atomic lay- PHYSICAL REVIEW B 74, 172411 2006 1098-0121/2006/7417/1724114 ©2006 The American Physical Society 172411-1